1. Field of the Invention
The present invention relates to an apparatus and method for withdrawing and dewatering slag from a gasification system. In particular, the present invention relates to a conveying lockhopper and method of using the conveying lockhopper with a gasification system.
2. Related Art
Gasification is among the cleanest and most efficient technologies for the production of power, chemicals and industrial gases from hydrocarbon feedstocks, such as coal, heavy oil, and petroleum coke. Gasification converts hydrocarbon feedstocks into clean synthesis gas, or syngas, composed primarily of hydrogen (H2) and carbon monoxide (CO). Gasification allows refineries to self-generate power and produce additional products. Thus, gasification offers greater efficiencies, energy savings, and a cleaner environment. For example, a gasification plant at a refinery in El Dorado, Kans. converts petroleum coke and refinery wastes into electricity and steam, making the refinery entirely self-sufficient for its energy needs and significantly reducing waste and coke handling costs. For these reasons, gasification has increasingly become popular among refineries worldwide. Currently, there are several hundred gasification plants in operation worldwide.
In a gasification plant, the feedstock is mixed with oxygen (O2) and injected into a gasifier. Inside the gasifier, the feedstock and the O2 are subjected to high temperatures and pressures that cause the feedstock and O2 to react and form syngas. Non-gasifiable ash material forms molten slag as a byproduct. Hot syngas exiting the gasifier is cooled either by direct contact with water in a quench chamber, or indirectly in a syngas cooler to recover excess heat/energy. In the direct quench mode, the hot syngas exiting the gasifier contacts water in the quench chamber located in the bottom of the gasifier vessel. The gas is cooled and saturated with steam. The molten ash is rapidly cooled and solidified into irregularly-shaped particles of varying size. Therefore, the quench chamber serves not only to cool and saturate the syngas, but also to disengage slag particles from the syngas, capturing the slag particles in the quench water.
In the syngas cooler mode, hot gas is indirectly cooled in a specifically designed heat exchanger where high pressure export steam is generated. At the exit of the syngas cooler, the cooled syngas is forced to make a rapid direction change as it passes over a pool of water called the syngas cooler sump. Molten and partially solidified slag entrained in the cooled syngas is thus disengaged from the syngas by the combined effects of gravity and momentum. The slag particles which drop down into the syngas cooler sump water then rapidly solidify into irregularly-shaped particles of various sizes.
In addition to H2 and CO, the syngas contains other gases in small quantities, such as carbon dioxide (CO2), water, ammonia, methane, hydrogen sulfide (H2S), carbonyl sulfide (COS), nitrogen, and argon. As much as 99 percent or more of the H2S and COS present in the syngas can be recovered and converted to elemental sulfur for use in the fertilizer or chemical industries. The clean syngas is then used for generating electricity and producing industrial chemicals and gases.
Most of the ash present in the solid feedstock is removed from the gasifier as solid, glass-like slag particles through a water-sealed, depressurizing lockhopper system. The lockhopper is conventionally a cylindrical vessel vertically oriented with top and bottom valves. This vessel is located directly beneath the gasifier quench chamber or syngas cooler sump.
The conventional lockhopper cycles through collection and dump modes. In the collection mode the top valvexe2x80x94the lockhopper inletxe2x80x94is open to the gasifier, and the bottom valvexe2x80x94the lockhopper outletxe2x80x94is closed. The entire lockhopper is filled with water forming a continuous column of water with the quench water, in the quench chamber mode, or with the syngas cooler sump water, in the syngas cooler mode. Thus, during the collection mode, slag entering either the quench chamber or the syngas cooler sump is able to drift unhindered downwards through the lockhopper inlet valve and into the lockhopper vessel.
Slag collection from the gasifier usually lasts for a period of 15-30 minutes. In the dump mode, the inlet is closed and the lockhopper is depressurized. After being depressurized, the bottom valvexe2x80x94the lockhopper outletxe2x80x94is opened for a short period of time, typically only a few seconds. During this period, the slag is flushed out with a deluge of water from a large overhead flush water tank. The large volume of water flushes the slag out of a conventional lockhopper either into a slag sump with a drag conveyor, or onto a slag pad. Conventional lockhoppers rely on slag sumps, drag conveyors, and slag pads to dewater the slag and prepare it for transportation.
If a slag sump is used, the drag conveyor slowly drags the water-soaked slag up a long incline by means of a series of heavy-duty scrapers connected by parallel chains in an arrangement that looks like a ladder. The drag conveyor circulates through the slag sump much like an escalator. The slag is dewatered by gravity-driven drainage as it get dragged up the incline over the top into a slag bin. In the case where the slag is flushed out of a conventional lockhopper onto a slag pad, the slag is drained of water by gravity as it sits atop the inclined concrete pad. Periodically, a front end loader is used to scoop-up dewatered slag and to transfer it to a slag bin for handling off site. Regardless of whether a slag sump/drag conveyor or a slag pad is used, the water which drains by gravity from the slag contains fine particles of slag which do not remain with the coarser material. This slag water is collected in a slag water sump from which it is pumped and processed as a dilute slurry for additional separation or filtration. Slag sump/drag conveyors, slag pads, and similar equipment usually require that large foundations be excavated in the earth beneath the gasifier and lockhopper so that the equipment can be positioned at the bottom of the lockhopper.
Since the lockhopper is a large elongated cylindrical vessel with top and bottom valves located directly beneath the gasifier, the gasifier structure must be supported high in the air. Because of the considerable weight and size of industrial gasification equipment, the need for structure to support that equipment at an increased elevation adds substantial expense to the gasification process. Because of its height, the elevated gasifier is more difficult to service.
In addition to the inefficiencies associated with supporting an elevated gasifier, the amount of water needed to flush the slag out of a conventional lockhopper during each dump cycle is typically 2-4 times the total volume of the lockhopper. This requires a considerable amount of processing equipment such as a slag drag conveyor or sump system to dispose of the slag and the water. Also, equipment is required to store intake water and to process and dispose of the resulting wastewater. Wastewater must be treated to meet environmental quality standards. The process of treating wastewater and adding new intake water is extremely costly. Additionally, to remove ammonia from the water within the system, an expensive reflux stripper is typically required. In some systems, even after the removal of the ammonia and a large percentage of contaminants, the purged water still does not meet strict environmental regulations.
For these reasons, a need has been recognized by the inventors for an improved lockhopper that can withdraw and process slag from a gasifier without the normal associated equipment and expense involved in supporting large gasification equipment above a large vertical lockhopper, and without expensive and wasteful water treatment processes and equipment.
The present invention relates to an apparatus and method for withdrawing and dewatering slag from a gasification system. In particular, the present invention relates to a conveying lockhopper and method of using the conveying lockhopper with a gasification system. The present invention solves the problems with, and overcomes the disadvantages of, conventional gasification systems.
In one aspect of the present invention, an apparatus for withdrawing and dewatering slagxe2x80x94referred to herein and by one skilled in the art as a lockhopperxe2x80x94is disclosed. The lockhopper of the present invention is an inclined cylindrical pressure vessel with a rotating auger or equivalent conveying device inside. This new lockhopper design, called a xe2x80x9cconveying lockhopper,xe2x80x9d is inclined, preferably at an angle of between approximately 5 and 60 degrees from the horizontal. The housing of the conveying lockhopper has an inlet and an outlet, both preferably controlled with valves. The inlet is configured to receive slag from a gasifier and the outlet is configured to discharge the processed slag directly into a container suitable for carrying the slag away. The conveying lockhopper is inclined, with the outlet preferably at a higher elevation than the inlet. The rotating auger or other suitable conveying device inside the housing conveys the slag from the inlet to the outlet. The length of the auger and the housing, the pitch of the auger flights, and the auger""s rotational speed can all be varied within the purview of the present invention to discharge slag from the conveying lockhopper at the desired rate. Since only a small portion of the conveying lockhopper is disposed beneath the gasifier, the elevation of the gasification system equipment, and consequently the amount of structure needed to support that equipment, is greatly reduced.
In another aspect of the invention, an improved gasification process is provided. Generally, the gasification process comprises supplying feedstocks and O2 or air to the gasifier through an injector, producing syngas which is cooled in a quench chamber, syngas cooler, or other syngas cooling apparatus, passing slag byproduct to the conveying lockhopper, then processing and discharging the slag through the conveying lockhopper. The conveying lockhopper operates in alternating collection and dump modes. In the slag collection mode, the outlet valve of the conveying lockhopper is closed and the inlet valve is opened so that slag can continuously pass from the gasifier into the conveying lockhopper through a continuous column of water that extends between the syngas cooling device and the conveying lockhopper.
Since the inlet valve from the gasifier remains open during this mode, the pressure within the conveying lockhopper is the same pressure as the pressure within the gasifierxe2x80x94typically about 500 pounds per square inch gage (psig), although it may range anywhere from 300 to 1500 psig. During the collection mode, which typically lasts 10-30 minutes, the auger is constantly rotating to breakdown the slag, separate it from the water, and convey it towards the outlet. In dump mode, the inlet valve is closed and the auger is stopped. The water level within the conveying lockhopper is reduced by pushing the water into an atmospheric heel tank using a pressurized cap of nitrogen. After the water has been pushed into the heel tank, the heel tank valve is closed and a vent valve is opened to depressurize the conveying lockhopper. After depressurization, the outlet valve is opened for a short time, typically 1-2 minutes, and the auger is restarted to discharge the slag. The dewatered slag is then discharged through the outlet valve and deposited into awaiting containers suitable for transporting the slag. In preparation for the next collection cycle, the conveying lockhopper is then refilled with water from the heel tank and pressurized to the gasifier operating pressure using an inert gas such as nitrogen. By cycling water in and out of the heel tank, the need for wasteful flushing of the lockhopper, and the corresponding costly equipment to do so, is eliminated.
In another aspect of the invention, the operation of the gasification process, particularly the operation of the conveying lockhopper, is computer controlled. A processor or other type of computer system is coupled to the conveying lockhopper to control the operation in collection and dump modes.
In yet another aspect of the invention, an apparatus for moving material is provided. The apparatus comprises an inclined conveyor with an inlet and an outlet, wherein the material is introduced to the conveyor by way of the inlet, the conveyor transports the material toward the outlet, and the material is expelled from the conveyor through the outlet.
In another aspect of the present invention, a pipe, pipe spool, or other type of storage buffer is attached to the outlet of the conveying lockhopper to act as a buffer for storing dewatered slag prior to discharge. The addition of a pipe spool ensures that dewatered slag does not collect at the discharge end of the conveying lockhopper where it could jam rotating seals and hamper auger operation. Utilizing a pipe spool may allow the auger to operate at a higher rotational speed, thus reducing the amount of torque vis-à-vis auger shaft speed required from the motor. Moreover, embodiments employing a pipe spool allow the conveying lockhopper to continuously collect and process slag while dewatered slag is being purged from the pipe spool.
Accordingly, the present invention provides an apparatus and method that eliminate costly excess structure, equipment, and processes for removing slag from a gasification system.
The present invention greatly reduces the height to which the gasifier must be elevated in order to position the lockhopper for use with the gasifier. As a result, the need for heavy duty structure to support an elevated gasifier is significantly reduced.
The present invention eliminates the need for a drag conveyor or a slag sump system to withdraw and process slag from the lockhopper. Consequently, there is no need for a drag conveyor or slag sump system, and no need to excavate the surface beneath the gasifier and lockhopper to accommodate such equipment.
The present invention is more cost effective and efficient because substantial amounts of processing equipment associated with the conventional lockhopper drag conveyor and slag sump water systems are eliminated. The installed cost of a slag handling system is, therefore, significantly reduced.
The present invention simplifies the process of disposing of dewatered slag by providing for the discharge of dewatered slag directly into transportable containers, eliminating the need for manpower and maintenance intensive operations associated with preparing processed slag for transportation.
The present invention significantly reduces the transportation cost for disposing of discharged slag from the lockhopper because the overall weight of the dewatered slag is lowered.
The present invention simplifies slag processing operations such as slag washing or pH adjustment for lowering the -concentration of leachable metals on the slag.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned in practice of the invention.